Medical electrodes provide an electrical interface between a patient and monitoring equipment (e.g., an electrocardiograph device) or between a patient and stimulating equipment (e.g., interferential and iontophoresis devices). A specific type of stimulating electrode, used to provide an electrical interface between a patient and defibrillation equipment, must be capable of conducting the high-energy level required for defibrillation. The present invention focuses on high-energy defibrillation and pacing electrodes. The general characteristics of, and distinctions among, monitoring electrodes, general stimulating electrodes, and defibrillation electrodes are outlined below.
A. Monitoring Electrodes
Medical monitoring electrode systems help to obtain desired physiologic responses for the assessment or treatment of diseases and injuries in humans. Monitoring electrodes are used to sense electrical signals, which are then transmitted to electrocardiograph (EKG), electroencephalograph (EEG), and electromyograph (EMG) devices. In general, monitoring electrodes for EKG, EEG, and EMG devices are small, for example on the order of a few square centimeters, because a relatively small contact area with a skin surface is sufficient for reception of electrical signals. Monitoring electrodes need only carry very low electrical signals: on the order of milliamps. In general, monitoring electrodes are not capable of conducting and distributing the high levels of energy required in transcutaneous stimulation and defibrillation electrodes.
B. Stimulating Electrodes
Stimulating electrodes emit electrical pulses for transcutaneous electrical devices, such as transcutaneous electrical nerve stimulation (TENS), electrical muscle stimulation (EMS), neuromuscular stimulation (NMS), functional electrical stimulation (FES), as well as interferential and iontophoresis therapy. Like monitoring electrodes, medical stimulating electrodes are also used to treat diseases and injuries in humans. Unlike and in contrast to monitoring electrodes, however, stimulation electrodes generally require a larger skin surface contact in order to provide sufficient transcutaneous electrical current to effect a desired physiologic response.
Many devices are designed for lower-energy level stimulation applications alone, such as TENS, EMS, NMS, FES, and interferential and iontophoresis therapy. At least some stimulation electrodes are touted as combination electrodes, which can also function as high-energy level defibrillation electrodes. U.S. Pat. No. 5,824,033 issued to Ferrari (“Ferrari”) discloses a disposable, multifunction (stimulating or defibrillating), x-ray transmissive electrode capable of conducting energy sufficient for defibrillation and having improved current density distribution between the electrode and the skin of the patient. See column 2, lines 7-13, of the Ferrari patent. Ferrari notes that monitoring electrodes are incapable of conducting and distributing the high levels of energy required in transcutaneous stimulation and defibrillation electrodes; thus, an important distinction exists between high-energy stimulating or defibrillating electrodes and lower-energy stimulating or monitoring electrodes. See column 1, lines 29-32. electrode disclosed in Ferrari includes an electrically conductive metal-metal chloride (e.g., silver-silver chloride) coating applied to one side of a sheet electrode member. See column 3, lines 31-41. Ferrari teaches that the sheet electrode as coated with the electrically conductive metal-metal chloride is not alone capable of transmitting and distributing the high levels of energy encountered in defibrillation over the entire surface of the electrode member. See column 4, line 66 to column 5, line 4. Thus, a current distributing mat is required and is adhered to the opposite side of the sheet electrode member.
The electrode member is a thin, flexible sheet of electrically conductive polymer film having a thickness of two to four mils (0.05 to 0.10 mm). The metal-metal chloride ink is applied in a layer or layers, by silk screening, and is preferably less than ten microns in thickness. See column 4, lines 17-30. The ink may be up to 1 mil (0.0254 mm) thick. The silk screen technique of applying the ink coating facilitates the application of multiple layers having different shapes and edge configurations to achieve a tiered effect. See column 10, lines 10-23.
The outer perimeter of the metal-metal chloride coating is spaced inward from the perimeter of the electrode member and outward from the perimeter of the mat. The metal-metal chloride coating is preferably formed in two layers, each a few microns in thickness. In addition, the layers are serrated or undulated at their outer perimeter. See column 6, lines 12-45.
The electrical conductors in the Ferrari electrode are multi-strand metal wires in which the unsheathed end portions are strands that are spread out and fanned as shown in FIGS. 1 and 3 of Ferrari. The fanned ends are bonded to the surface of the mat by pressing them against the mat and folding the mat over the ends. Specifically, the wires are metallized carbon fiber tows with a metal (e.g., nickel or copper) coating. See column 6, line 46 to column 7, line 40.
C. Defibrillation Electrodes
In a malady called “fibrillation,” the normal contractions of a muscle are replaced by rapid, irregular twitchings of muscular fibers (or fibrils). Fibrillation commonly occurs in the atria or ventricles of the heart muscle; the normal, rhythmical contractions of the heart are replaced by rapid, irregular twitchings of the muscular heart wall. A remedy for fibrillation is called “defibrillation,” a procedure which applies an electric shock to arrest the fibrillation of the cardiac muscle (atrial or ventricular) and restore the normal heart rhythm. A system of two electrodes, one positive and one negative, is typically used to apply the electrical potential in a defibrillation procedure.
Defibrillation electrodes must be capable of conducting the high-energy level required for defibrillation, up to 360 Joules or more. Defibrillation electrodes must also distribute the energy over a relatively large area of the epidermis of the patient to achieve adequate current density distribution within the atria or ventricles. These characteristics are sufficiently important that governmental regulatory agencies and medical industry groups have established standards for defibrillation electrodes. In particular, the American National Standards Institute (ANSI) standards for defibrillation electrodes have been published by the Association for the Advancement of Medical Instrumentation (AAMI). The ANSI standards for the size of defibrillation electrodes recommend, for example, that the minimum active area of individual, self-adhesive electrodes used for adult defibrillation and pacing shall be at least 50 cm2 and that the total area of the two electrodes shall be at least 150 cm2.
U.S. Pat. No. 5,352,315 issued to Carrier et al. is directed to a biomedical electrode, suitable for defibrillation, that uses a conductive ink to provide varying impedances and at the same time is inexpensively produced and disposable as well. The conductive ink layer or layers may be of the silver and silver chloride type and may be applied by screen printing. The disclosed embodiments provide for the ink blends and ink amounts (i.e., ink thickness and ink pattern) to be varied so that the thickness and pattern provide a particular impedance value suited for the intended placement of the electrode at a particular body site.
A perspective view of another conventional defibrillation electrode construction is shown in FIGS. 2A and 2B. In general, the electrode comprises a sheet electrode member 202 of electrically conductive, carbon-filled polymer; an electrically conductive metal/metal chloride coating 204 (and preferably a silver/silver chloride coating) on at least a major portion of the lower side of the electrode member 202; and a pad of electrically conductive gel 206 underlying the metal/metal chloride coating 204 on the lower side of the electrode member 202. A removable release carrier sheet 208, for example of silicone-coated polyethylene terephthalate (PET), underlies the gel pad 206 and covers the latter before use. The electrode is configured to be x-ray transparent and capable of conducting electrical energy at levels sufficient for defibrillation. The phrase “x-ray transparent” is defined as the quality of being at least substantially invisible at x-ray irradiation levels used in routine x-rays of a patient's chest.
The electrode member 202 is formed of a thin, flexible sheet of electrically conductive polymer film such as graphite-filled polyvinyl chloride film preferably having a thickness of the order of two to four mils (0.05 to 0.10 mm). An example of carbon-filled polymer which can be used is thin, carbon-filled polyvinylchloride (PVC) available from Burkhardt/Freeman, Holyoke, Mass., under the trademark “Conducton.” The electrode member 202 has a tab portion 210 with an aperture 212.
The electrode member 202 has a surface area dimensioned to distribute energy over an area of the patient's epidermis to achieve proper current density distribution within the ventricles of the patient's heart. The ANSI standards for the size of defibrillation electrodes published by AAMI recommend that the minimum active area of individual, self-adhesive electrodes used for adult defibrillation and pacing shall be at least 50 cm2 and that the total area of two electrodes used in defibrillation shall be at least 150 cm2. The electrode member 202 has an area of at least 50 cm2 and preferably about 80 cm2 or more so that a pair of the electrodes used for defibrillation can be of the same size.
The coating 204 of metal/metal chloride is typically a conductive ink layer comprising a galvanic metal such as silver, and a conductive salt such as silver chloride. The coating 204 is applied in a layer or layers to the lower face of the electrode member 202 by silk screening or by flexographic printing. A carbon-filled PVC material with silver/silver chloride coating on the underside suitable for use as an electrode member is available from Prime Label And Screen, Inc., New Berlin, Wis. Alternatively, the metal/metal chloride coating 204 can comprise a single layer, chloride-coated metallic foil coated with a conductive acrylic adhesive. The metallic foil may comprise silver, tin, copper, nickel, gold, aluminum, platinum, chromium, cadmium, palladium, zinc, antimony, or indium covered with an adhesive such as the Arclad 8001 bonding tape or Arclad EC2 adhesive. An aperture 214 is provided in the coating 204 and positioned to align with the aperture 212 in the electrode member 202.
An electrolytic gel pad 206 underlies the metal/metal chloride coating 204 on the lower surface of the electrode member 202. The gel pad 206 is preferably a skin-compatible hydrogel having good ability to retain moisture content and adhesive tack. The gel pad 206 is of a type that adhesively connects the electrode to the patient's skin. The gel may comprise, for example, a hydrogel marketed by Ludlow Technical Products (a division of Tyco International Corporation) under the trademark “Procam,” product number GRG73P.
At the head 216 of the gel pad 206 are provided a pair of foam tabs 218, and 220. One of the tabs 220 is covered with an adhesive 222. An energy conductor 224 such as a conductive post, stud, or rivet is conductively adhered to the electrode construction. The conductor 224 aligns with, and passes through, both the aperture 214 in the coating 204 and the aperture 212 in the electrode member 202. Such a conductor 224 permits cost-effective use of the electrode with certain defibrillators currently on the market. The conductor 224 may be made of a conductive metal (such as nickel-plated brass or stainless steel) or a conductive plastic. The conductive plastic may be ABS plastic resin, nylon 12, or Carillon polymer crystal resin manufactured by Shell Oil, loaded with 25-40% nickelized carbon fibers. After being molded into its shape, the conductive plastic may be silver-coated (by, e.g., electrolysis) to further enhance its conductivity.
As shown in FIG. 2B, an oversized cover sheet 226 having an adhesive layer on its lower surface is secured to the top of the electrode member 202 (not visibly shown). The cover sheet 226 is x-ray transparent and made of electrically insulative foam such as 0.08 to 0.16 cm thick polyethylene (PE) foam. Shown in FIG. 2B are the two electrodes that form a defibrillation pair of pad electrodes, with cover sheet 226 forming the right pad and cover sheet 228 forming the left pad. The components underlying each of cover sheet 226 and 228 are illustrated in FIG. 2A and discussed above. Cover sheet 226 has an aperture 230 and cover sheet 228 has an aperture 232. Each aperture 230 and 232 aligns with both the aperture 212 in the coating 204 and the aperture 212 in the electrode member 202 respectively underlying the cover sheets 226 and 228 and receives a respective conductor 224. Because the electrodes are x-ray transparent, they can be positioned on the patient at any of the customary positions used for defibrillation without adversely affecting x-rays of the patient's chest in areas underlying the electrodes.
As diagrammatically shown in FIG. 2B, the energy-delivery and energy-accepting electrodes, represented by their respective cover sheets 226 and 228, are connected through conductors 234 and 236 to a connector 238. The connector 238 engages a corresponding connector 240 having lead conductors 242 and 244 which are connected, in turn, to a defibrillator 246. Conductors 234 and 236 of connector 238 are mechanically and electrically connected to the respective energy-delivery and energy-accepting electrodes, through the conductor 224 of each electrode, using a conductive ring contact 248 and a foam ring 250.
The carbon-filled polymer electrode member 202 is conductive in the plane of the electrode and transverse to the plane of the electrode and the metal/metal chloride coating 204 on the under side of the electrode member 202 is also conductive in the plane of the coating and transverse to the plane of the coating. The carbon-filled polymer electrode member 202 has a surface resistance substantially higher than the surface resistance of the metal/metal chloride coating 204 and it has been found that the carbon-filled polymer electrode member 202 with a silver/silver chloride coating 204 is not alone capable of transmitting and distributing the high levels of energy encountered in defibrillation over the entire surface of the electrode member 202.
In addition, published literature indicates that, when a metal plate electrode having an electrolytic gel coating on its underside is placed on the skin and used to deliver current, the current density is very much higher under the perimeter of the electrode than under the center. A similar problem occurs at the energy-accepting electrode of a set of such defibrillation electrodes.
The conventional defibrillation and stimulating electrodes of FIGS. 2A and 2B utilize an electrically conductive metal-metal chloride (i.e. silver-silver chloride) coating 204 applied to one side of a sheet electrode member 202. This design suffers from several shortcomings, including polarization of the electrodes. Upon complete depletion of either metal chloride (i.e. silver chloride) on the negative electrode or metal (i.e. silver) on the positive electrode, electrolysis of water present in the gel pad 206 will begin. Consequently, an acid will be produced at the positive electrode and an alkali will be produced at the negative electrode in the form of H+ and OH−, respectively. These acid and alkali components are then iontophoretically driven into the skin of a patient by current flow, and the result can cause burning of the skin.
It is desirable to provide a mechanism by which the polarization of the electrodes can be resisted, and further, to provide a mechanism to resist the effects of polarization of the electrodes.